N-Protected 2-oxa-3-azabicyclo[2.2.1]hept-5-enes are valuable intermediates in the synthesis of various pharmaceutically active ingredients. See e.g. EP-A-0 322 242 and EP-A-0 658 539 for the N-benzyloxycarbonyl derivative. While some racemic compounds are relatively easily obtainable by hetero-Diels-Alder cycloaddition of nitroso compounds such as benzyl nitrosoformate (obtainable from benzyl N-hydroxycarbamate by oxidation, e.g. with periodate) with cyclopentadiene, a commercially feasible method for the production of the enantiopure or enantiomerically enriched compounds with a wide variety of possible protective groups has not been available.